Zero-crossing is a type of frequency division system that counts every time a sound wave crosses an imaginary zero point and provides a frequency / time output. Zero--crossing detectors are primarily used to record bat calls for analysis with bioacoustics software and as they produce small file sizes are efficient with battery useage they can be left in the field unattended for long term monitoring. They only capture the loudest frequency at any one time, however, so harmonics are not recorded and the output does not contain amplitude information.
